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what size PEX should i use 1/2 or 3/4 to replace 3/4 copper that replaced 1/2 galvanized the copper is fairly new 11yrs? but is laying on the ground and there is still 1/2 galvanized going to all fixtures i want to replace this hodgepodge with PEX but am baffled by what size to use

You need to give out more information. Pipe sizes are based on the number of fixtures they supply and the length of the run, etc. If anyone has some general rules of thumbs on this, I would like to hear them also. I always replace a 3/4" pipe with a 3/4" pipe no matter what the material (if there are no other problems). Never plumb a pipe to lay on the ground.